Damn. It turns out that the standard wkhtmltopdf package needs the server to have a window manager running and the chances are your server, like this one, doesn’t have a GUI interface. So what now? Well after a bit of Googling I was starting to get worried, until somebody (I forget where, but thank you!) mentioned that the binaries that can be downloaded from the project’s repo are built against qt and don’t depend on X running! Result!